(1) Field of the Invention
This invention relates to an ink jet printer for recording patterns of letters, figures, etc. with ink droplets and an ink droplet projecting device used in this printer.
(2) Description of the Prior Art
An ink jet printer of non-impact type has excellent features such as emission of noise at low level, applicability to the ordinary kind of paper, and feasibility of easy color printing. Ink jet printers now in practical use are various in type such as electric charge control type, electric field control one, on-demand one, and ink-mist one, among which the on-demand type is widely used because of advantages thereof that unused ink need not to be withdrawn, coloring is quite easy, and color shade can be varied by altering the diameter and the number of ink droplets. Generally, such on-demand type ink jet printer, particularly the ink droplet projecting device thereof, is further classified into two types: one being the so-called "non-air-flow type" provided with an ink projecting head having an ink cell furnished with an orifice and a vibrating plate on the peripheral wall thereof for projecting ink contained in the ink cell through the orifice toward the recording paper by vibrating said vibrating plate with imparting the electric signal thereto; and the other being the so-called "air-flow type" in which an air cell provided with the 2nd orifice (air jetting orifice) positioned in front of the ink cell and facing the 1st orifice of said ink cell is formed so that pressurized air is blown from this air cell through said air jetting orifice toward the recording paper for adapting ink droplets projected through the 1st orifice to be spouted toward the recording paper. Either type, however, is followed by such drawbacks that, since the orifice opens to the air and consequently water content of the ink in the cell evaporates through the orifice during suspension of use of the ink projecting head, not only solid constituents of the ink stick to the periphery of the orifice and sometimes clog the orifice but also a reduced volume of ink as a result of water content evaporation causes suction of air into the ink cell and insufficient transmission of pressure wave from the vibrating plate to ink and, accordingly, irregular projections of ink droplets.
In view of these problems, several proposals as described under have hitherto been made but have not yet brought about satisfactory solutions.
(a) To put a lid over the orifice
(b) To keep the orifice and its ambient space in humid atmosphere
(c) To mix ink with anti-drying agents such as polyethyleneglycol, ethyleneglycol, etc.
(d) To remove solidifying ink sticking to the orifice by the application of high pressure to ink in the cell.